The authors of this recent PLoS ONE paper used long-term datasets of daily flow, temperature, and fish counts from Winchester Dam in southern Oregon to understand links between the phenology of upstream migration and environmental regimes. The authors used a new technique that synthesized large amounts of data related to fish migration on the North Umpqua River into what they call an “ichthyograph.” Their ichthyographs show the general patterns in upstream migration for six fish species native to the Northwest – steelhead, sucker, chinook salmon, lamprey, cutthroat trout and coho salmon. Authors hope that the ichthyographs will help fisheries managers assess the impacts of climate change or human-related activity such as water control and diversion, floodplain stabilization and road construction.

Growing consensus on climate and land use change means that it is reasonable to assume, at the very least, that flood levels in a region may change. Then why, ask Rosner et al. in a new study, do the dominant risk assessment techniques used to decide whether to build new flood protection infrastructure nearly always start with an assumption of “no trend” in flood behavior?

Commentary on Heightened Risk of Drought due to Climate Change

In this commentary piece, Pacific Institute Director Peter Gleick and Pennsylvania State University Meteorologist Michael Mann discuss the state of current literature on drought and climate science. They discuss a new study published in PNAS by Diffenbaugh et al., which shows accumulating evidence that climate change is influencing the frequency, magnitude and duration of drought in California. An increasing number of dry years along with warm years raise the risk of drought, despite the lack of a strong trend in precipitation. These results point to the significance of warming temperatures to changing the availability of water and increasing drought intensity. It is important to note, however, as the authors do, that this is not uncontested. A number of recent studies (some of which focused on a lack of trend in precipitation) concluded that a link between ocean temperatures and drought could not yet be established. Part of the debate, however, has occurred because there are many ways in which drought can be defined. A drought can be meteorological, hydrological, agricultural, and/or socioeconomic. Other parts of the debate stem from attribution. Some studies argue that low levels of precipitation cannot be tied to climate change, while others argue that while this may be true, the low levels of precipitation are caused by an unusually strong ‘atmospheric ridge’ in the Western United States, which was most likely stronger due to climate change.

Global volume and distribution of groundwater and its vulnerability to climate change

In this new study, scientists updated a 40-year-old estimate of the Earth’s total volume of groundwater. The study compiled geochemical, geologic, hydrologic, and geospatial datasets with numerical simulations of groundwater, as well as analyzed modern groundwater (less than 50 years old) from tritium measurements. Modern groundwater is important because it is; 1) a better renewable resource than older groundwater, 2) a huge component of the hydrologic cycle as well as global biogeochemical cycles, and 3) more vulnerable to industrial or agricultural contamination. This study distinguishes modern groundwater from older groundwater by measuring tritium, an isotope of hydrogen whose concentration in precipitation peaked approximately 50 years ago (during above-ground thermonuclear testing). The study found that less than 6% (0.1-5.0 million km3) of the groundwater in the uppermost portion of Earth’s landmass is modern. Despite seeming minor, the volume of modern groundwater is equivalent to a body of water with a depth of about 3 m spread evenly over the continents. This water resource dwarfs all other components of the active hydrologic cycle and will be critical for future energy, food security, human health, and ecosystems.

The mountain pine beetle infestation of the Rocky Mountains has resulted in unprecedented tree death across North America. Researchers analyzed contributions to streamflow over time and space to investigate the potential for increased groundwater inputs resulting from hydrological change following the mountain pine beetle infestation. Results of this study in Rocky Mountain National Park show that on average, groundwater fractions remain higher after infestation even when including the assumption that interannual differences in snow water equivalent translate directly to less streamflow. Additionally, results indicated that forest transpiration decreased after trees infected with the mountain pine beetle died. Understanding the impacts of mountain pine beetle infestation on the hydrological cycle provides necessary understanding for water resource management in mountain pine beetle infested forests.

Understanding the California drought as an extreme value:

This study uses the statistical technique of extreme value theory to understand the probability of how often a drought as severe as California’s is expected to occur. The author, Scott Robeson of the University of Indiana at Bloomington, draws upon the method of Griffin and Anchukaitis (2014) but shows how their method had an error because it did not account for the respective areas (in square kilometers) of Central and Southern California. He uses observational data from 1895 – 2014 on the Palmer Drought Severity Index (PDSI) from tree rings to estimate the occurrence of droughts beginning in the year 800. Robeson finds that the 2014 drought in California had a return period of 140-180 years, e.g. a probability of occurring every 140 – 180 years. However, if the drought from 2012 through 2014 is included, rather than just the year 2012, the probability decreases to a 10,000 year event, or occurring every 10,000 years. If this year is also included, the probability of the drought occurring decreases to zero. Hence they conclude that the drought is completely without precedent.

A new report details outcomes from a North Pacific Landscape Conservation Cooperative workshop held in January, 2014 that focused on changes in hydrologic regime on rivers, streams, and riparian corridors. The USGS helped to determine what information managers need in order to address the consequences of climate change on valued resources. [FullText] Contact: Andrea Woodward, 206-526-2534, andrea_woodward@usgs.gov

Drought indices are often used to monitor interannual variability in regional pattern of hydrology, but approaches vary widely. This study correlated various indices to water-year runoff for 21 unregulated drainage basins in the Pacific Northwest of the United States to identify those indices that explain the greatest amount of variability.

NWCSC Academic Director Phil Mote and colleagues recently published a report examining the “snow drought” of 2014-2015 in Washington and Oregon. The authors used a crowd-sourced superensemble of regional climate model simulations to compare human-induced causes to changes in sea surface temperature (SST) as contributions to the snow drought. Additionally, the study compared causes of the snow drought to the anomalous drought in California from 2011-2015. Findings from this study showed that SST anomalies contributed twice as much as anthropogenic effects, however both exhibited strong influences on the snow drought. Comparing this to the California drought, the authors conclude that both extreme events were exacerbated by human-induced rises in temperature.